My 2010 F 150 with a 4.6 3V was occasionally experiencing the
same exact thing with the key after it was released. Turned out
to be the EVAP solenoid pump on the top of the engine plus
another EVAP solenoid. The EVAP system on 2010 F 150
suppose to be warranted for 7 years. Have a dealership check it
out, it may cost nothing.